1. Field of Invention
Embodiments of the invention relate generally to methods and apparatus for providing uninterruptible power to sensitive and/or critical loads. More specifically, embodiments of the invention relate to distribution and control systems for power systems having backup power sources.
2. Discussion of Related Art
The use of uninterruptible power supplies (UPSs) and backup power sources, such as generators, to provide uninterrupted power to critical loads is well known. Typically, UPSs use batteries to provide uninterrupted power during relatively brief power outages, while backup generators are used to provide power during longer power outages. Typically, a UPS is used to provide power for outages less than thirty minutes, but depending on the power draw of a load, and the capacity of batteries used in a UPS, the actual runtime of a UPS may be greater or less than thirty minutes. When longer runtime than that provided by a UPS is required, a generator may be used. When generators are used, a UPS may still be used to provide power during short outages and to provide continuous power during the period of time required to start a generator and bring it on line after an outage has occurred.
The installation of a generator in a facility power system is typically an expensive ordeal requiring the hiring of an engineering firm along with several subcontractors to design and install the associated control and switching systems needed to integrate the generator into the facility power system. As a result, each system is a unique system that requires substantial labor in the field in an uncontrolled environment. Control systems are often designed into facility power systems, however, because different components of the systems may be supplied by different manufacturers, these control systems often use different communication protocols making communication among the components difficult, if possible at all.
In typical installations that employ a generator, a transfer switch is used to transfer a source of power for the installation between a primary source (utility) and a back-up source (generator). The transfer switches typically are implemented using a single device having a mechanical interlock that prevents power from being simultaneously supplied from both the primary source and the back-up source. One problem with these switches is that they represent a single point of failure in the system. Upon certain failure conditions of such transfer switches, since the switch is implemented using a single device, the switch is not able to provide power from either the primary source or the back-up source. In critical facilities, such failures are unacceptable. Another problem with these switches is that the mechanical interlock typically prevents simultaneous power from being supplied by both the generator and the utility, resulting in a brief power outage when utility power returns and the supply of power is switched from generator to utility.